So much for the concepts - let's get to Doing One! (you should probably download and print this diagram)
If you start with a rectangular blank, you've got square ends, square to the side reference surfaces. Layout is pretty straight forward - a pencil, a square and a compass.
But if you start wth a piece of a branch, or rough cut chunk of a tree, there are few if any flat surfaces to layout lines on. Transferring lines from one end of the blank to the other gets tricky, Probably an obvious solution but here's something that will make things easier.
If you set your tool rest height to the centerline of your lathe - then - scratch or file a line
where its shaft meets your banjo, you can reset to that height quickly and easily and use
your tool rest to draw lines down the sides - at the lathe's centerline height.